As a result of the increased concern over the dangers of carbon monoxide, many attempts have been made to provide a carbon monoxide detecting device that may be connected to a fuel fired heating appliance to provide for shutting off the appliance in the event harmful levels of carbon monoxide gas should accumulate. However, installing such devices could cause unnecessary shut off of the fuel fired heating appliance, as a result of the carbon monoxide sensor indicating a false level of carbon monoxide. Likewise, improper placement of the sensor could result in a failure to trigger shut down of the appliance even in the presence of harmful levels of carbon monoxide gas concentration. Furthermore, exposure to other chemicals or humidity may affect the output of the sensor, and can lead to difficulties in the interpretation of positive readings of the sensor.